Observation of low frequency gravitational waves from a space observatory such as eLISA — proposed for the ESA “Gravitational Universe” large mission (L3) theme selected for the 2030 timeframe — will revolutionize astrophysics, opening a whole new window of discovery for the physics and astronomy of massive black holes at galactic centers, stellar mass compact objects, and the interactions among and between these disparate astrophysical populations.

The gravitational waves produced by these distant sources produce an measurable time varying differential force that on a constellation of free-falling test particles, a “tidal deformation” that alternately shrinks and expands their effective separation. As for terrestrial gravitational wave observatories like LIGO and VIRGO, an orbiting observatory will detect the relative acceleration of the test particles using laser interferometry, in the minute variations of the Doppler shift of a light beam passing between the test particles.

In the proposed eLISA observatory, test masses – roughly kg-sized metal cubes – will orbit inside three spacecraft at the vertices of an orbiting triangle with side length of at least 1 million km, with the relative changes in the triangle side length measured with Michelson laser interferometry. The large interparticle separation – which amplifies the gravitational wave tidal acceleration – is only possible in space, which also isolates the test masses from the noisy gravitational forces near the Earth. The test masses play the role of “end mirrors” in the interferometer as well as references of purely free-falling geodesic motion, allowing the gravitational wave accelerations to dominate over any other “spurious” disturbance forces.

eLISA seeks to detect test particle relative acceleration at the level of 10^{-16} m/s^{2} or smaller – 17 orders of magnitude smaller than * g*, the gravitaitonal acceleration on the Earth changes in the test particle separation – in one cycle of a wave with periods from 100 to 10000 s. This corresponds, at the higher frequencies, to relative displacements of less than 1 picometer (1 pm or 10

^{-12}m), or relative changes in the triangle side length – known as strain – of order 10

^{-20}. These numbers represent the experimental challenge of gravitational wave astronomy.